Highly tackified acrylate pressure sensitive adhesives

ABSTRACT

The present invention relates to a radiation crosslinkable pressure sensitive adhesive precursor comprising:
         a) an acrylate base polymer;   b) a co-polymerized type (II) photocrosslinker, in a amount greater than 0.05 parts by weight per 100 parts by weight of acrylate base polymer;   c) a co-polymerized hydrogen-donating monomer; and   d) a tackifying resin, in an amount greater than 40 parts by weight per 100 parts by weight of acrylate base polymer.       

     The present invention is also directed to a method of preparing a radiation crosslinked pressure sensitive adhesive.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/442,874, filed May 14, 2015, now pending, which is a is a nationalstage filing under 35 U.S.C. 371 of PCT/US2013/068376, filed Nov. 5,2013, which claims priority to EP Application No. 12193296.6, filed Nov.19, 2012.

TECHNICAL FIELD

The present disclosure relates generally to the field of adhesives, morespecifically to the field of photocrosslinked pressure sensitiveadhesives which contain relatively high levels of tackifying agents. Thepresent disclosure is also directed to a method of preparing highlytackified photocrosslinked pressure sensitive adhesives.

BACKGROUND

Adhesives have been used for a variety of marking, holding, protecting,sealing and masking purposes. Adhesive tapes generally comprise abacking, or substrate, and an adhesive. One type of adhesive, a pressuresensitive adhesive, is particularly preferred for many applications.Pressure sensitive adhesives (PSA's) are well known to one of ordinaryskill in the art to possess certain properties at room temperatureincluding the following: (1) aggressive and permanent tack, (2)adherence with no more than finger pressure, (3) sufficient ability tohold onto an adherend, and (4) sufficient cohesive strength.

Materials that have been found to function well as pressure sensitiveadhesives are polymers designed and formulated to exhibit the requisiteviscoelastic properties resulting in a desired balance of tack, peeladhesion, and shear strength. The most commonly used polymers forpreparation of pressure sensitive adhesives are natural rubber,synthetic rubbers (e.g., styrene/butadiene copolymers (SBR) andstyrene/isoprene/styrene (SIS) block copolymers), various (meth)acrylate(e.g., acrylate and methacrylate) copolymers and silicones.

General purpose tapes which stick to all types of surfaces andespecially pressure sensitive adhesives which stick very well to LowSurface Energy substrates typically require addition of high amounts oftackifying resins. PSA's prepared from solution polymer may compensatefor the reduced cohesive strength, due to the presence of low molecularweight tackifying resin, with appropriate addition of crosslinkers orincreased molecular weight of the polymer. In hot melt processableformulations though, the polymer has to be able to flow sufficiently atextruder temperature and therefore the maximum molecular weight islimited. Furthermore, the combination with thermal crosslinkers tocreate a higher cohesive strength via an increase of the molecularweight and the creation of a chemical network is not always practicalbecause of its potential implications during hotmelt processing.

It is known that crosslinking of polymers produces polymer networkswhich have quite different mechanical and physical properties comparedto their uncrosslinked linear or branched counterparts. For example,polymer networks can show such unique and highly desirable properties assolvent resistance, high cohesive strength, and elastomeric character.

Crosslinked polymers can be made in situ during formation of the desiredpolymer product. Many patents are known describing techniques to achieveefficient crosslink mechanisms and good cohesive strength properties.Therefore, the problems associated with solvent and bulk processing ofcrosslinked materials may be avoided through the use of actinicradiation processing. U.S. Pat. No. 4,379,201 (Heilmann et al.)discloses an example of a class of polyacrylic-functional crosslinkersused in the photocuring of (meth)acrylate copolymers.

U.S. Pat. No. 4,391,678 (Vesley) and U. S. Pat. No. 4,330,590 (Vesley)describe a class of fast curing triazine photocrosslinkers which, whenmixed with an acrylic monomer and, optionally, a monoethylenicallyunsaturated monomer, and exposed to UV radiation, forms a crosslinkedpolyacrylate. The crosslinks formed by both the (meth)acrylates and thetriazines in these copolymerizations prevent any further processing,such as hot melt coating, reactive extrusion, or solution coatingprocesses, following the initial photopolymerization. However, sincefurther processing of the polymer product is often necessary, it is moretypical to start from the linear or branched polymer which, in the finalprocessing step, is cured to a crosslinked material. The curing orcrosslinking step is typically activated by moisture, thermal energy oractinic radiation. The latter has found widespread applications,particularly in the use of ultraviolet light as the radiation source.

In the past, a variety of different materials have been used ascrosslinking agents using actinic radiation, e.g. polyfunctionalacrylates, acetophenones, benzophenones, and triazines. The foregoingcrosslinking agents may however possess certain drawbacks which includeone or more of the following: high volatility; incompatibility withcertain polymer systems; generation of corrosive or toxic by-products;generation of undesirable color; requirement of a separate photoactivecompound to initiate the crosslinking reaction and high sensitivity tooxygen.

In addition to actinic radiation processing described above, acrylatePSAs can be applied to substrates by solvent and hot-melt coatingtechniques. Although solvent coating techniques are widely used,hot-melt coating techniques may provide some environmental andeconomical advantages. However, unlike solvent coating techniques wherethe polymer coating and crosslinking are performed simultaneously,hot-melt coating requires that coating and crosslinking be performedsequentially. This is due to competing considerations: a polymer shouldnot be highly crosslinked if it is to be hot-melt coated smoothly, yetthe polymer needs to be crosslinked to achieve certain desirableperformance properties such as e.g. high shear when the polymer is aPSA. Therefore, hot-melt coating is performed prior to crosslinking ofthe coated polymer.

Because hot-melt coating techniques involve high amounts of thermalenergy and shear, the subsequent crosslinking procedure usually involvesnon-thermal energy sources. Electron beam (e-beam) and ultraviolet (UV)energy sources have been used traditionally, although e-beam techniquesoften are too energy intensive to be practical. Accordingly, muchinterest has been focused on UV radiation crosslinking of polymers.

UV radiation crosslinking of coated polymers has relied almostexclusively on hydrogen abstraction techniques in which a hydrogenabstracting agent, such as e.g. benzophenone or anthraquinone, isblended into the coated mixture prior to or during the coating process,and the mixture is then exposed to appropriate UV radiation. Certainpolyfunctional benzophenones have been investigated as photocrosslinkingagents and/or photosensitizers in various photopolymerizable systems.

To date, there is no disclosure of any highly tackified radiationcrosslinkable PSA formulations, in particular solventless PSAformulations, which include an incorporated photocrosslinker at levelshigher than 0.05 parts by weight per 100 parts of polymeric material,and which after suitable radiation crosslinking provides a highlytackified radiation crosslinked pressure sensitive adhesive inparticular provides high cohesive strength at elevated temperature andhigh-temperature shear resistance whilst ensuring excellent adhesion tovarious types of substrates.

Also, when a tackifying resin is present in the PSA formulation,especially in a relatively high amount, a large fraction of the exposedUV light during the crosslinking step is absorbed by the tackifyingresin/photocrosslinker system which may result in reduced crosslinkingefficiency and poor cohesive strength of the resulting PSA. Whenradiation crosslinking is used to crosslink tackified PSA formulations,the tackifying resin may provoke several other deleterious effects suchas e.g. undesired chain transfer or chain termination reactions. The useof high levels of tackifying agent(s) may be desirable because it canincrease the tackiness of the pressure sensitive adhesive, making itaggressively adhere to wide range of substrates without the need toapply pressure. The addition of tackifying resin, especially high levelsof tackifying resin, may detrimentally affect the shear and cohesivestrength of a pressure sensitive adhesive, and may even raise the Tg ofthe adhesive. The use of high levels of tackifying resin may beparticularly detrimental to hot melt processable pressure sensitiveadhesives where the need to be hot melt processable can alreadyadversely affect the shear strength and cohesive strength properties ofthe adhesive.

U.S. Pat. No. 4,737,559 (Kellen et al.) discloses a PSA which is acopolymer of an acrylate monomer and a copolymerizablemono-ethylenically unsaturated aromatic ketone comonomer free ofortho-aromatic hydroxyl groups. WO-A1-97/40090 (Stark et al.) describesa radiation crosslinkable composition comprising: a) a radiationcrosslinkable polymer having abstractable hydrogen atoms andradiation-activatable crosslinking groups capable of abstractinghydrogen atoms when activated; and b) a non-polymerizableradiation-activatable crosslinking agent capable of abstracting hydrogenatoms when activated. WO-A1-96/35725 (Carpenter) discloses pigmented,UV-crosslinked, acrylic-based, pressure sensitive adhesives claimed tohave high cohesive strength and high-temperature shear resistance. Theadhesives disclosed in WO-A1-96/35725 comprise an acrylic copolymercompounded with a pigment and a hydrogen-abstracting photoinitiator,wherein the acrylic copolymer is obtained by copolymerizing an alkylacrylate and a tertiary amine-containing monomer. WO-A1-2012/044529(Satrijo et al.) describes a hot-melt processable PSA comprising: a) ahot-melt processable elastomeric (meth)acrylate random co-polymer; b) atleast one tackifing resin comprising greater than 50 parts by weight per100 parts by weight of elastomeric (meth)acrylate random co-polymer; andc) a thermoplastic material.

Without contesting the technical advantages associated with thesolutions disclosed in the art, there is still a need for a highlytackified radiation crosslinked pressure sensitive adhesive whichovercomes the deficiencies previously mentioned, and which in particularprovides high cohesive strength at elevated temperature andhigh-temperature shear resistance whilst ensuring excellent adhesion tovarious types of substrates.

Other advantages of the pressure sensitive adhesives, precursors ofpressure sensitive adhesive and methods of the invention will beapparent from the following description.

SUMMARY

According to one aspect, the present disclosure relates to a radiationcrosslinkable pressure sensitive adhesive precursor comprising:

-   -   a) an acrylate base polymer;    -   b) a co-polymerized type (II) photocrosslinker, in a amount        greater than 0.05 parts by weight per 100 parts by weight of        acrylate base polymer;    -   c) a co-polymerized hydrogen-donating monomer; and    -   d) a tackifying resin, in an amount greater than 40 parts by        weight per 100 parts by weight of acrylate base polymer.

According to another aspect, the present disclosure relates to a methodof preparing a radiation crosslinked pressure sensitive adhesive,comprising the steps of:

-   -   a) providing a radiation crosslinkable pressure sensitive        adhesive precursor as above-described; and    -   b) radiation crosslinking the radiation crosslinkable pressure        sensitive adhesive precursor.

In still another aspect, the present disclosure relates to the use of aradiation crosslinkable pressure sensitive adhesive precursor as abovedescribed, for the manufacture of an adhesive tape, preferably adouble-sided tape.

DETAILED DESCRIPTION

The present invention provides a highly tackified radiation crosslinkedpressure sensitive adhesive which is, in particular, provided with highcohesive strength at elevated temperature whilst ensuring excellentadhesion to various types of substrates, in particular low surfaceenergy substrates. In particular, the present invention providesversatile highly tackified radiation crosslinkable PSA formulations, inparticular solventless PSA formulations, which include an incorporatedphotocrosslinker at levels higher than 0.05, in particular greater than0.10, more in particular greater than 0.15 parts by weight per 100 partsof polymeric material, and which after suitable radiation crosslinkingstep produces highly tackified pressure sensitive adhesives havingbeneficial properties.

Accordingly and in a first aspect, the present invention relates to aradiation crosslinkable pressure sensitive adhesive precursorcomprising:

-   -   a) an acrylate base polymer;    -   b) a co-polymerized type (II) photocrosslinker, in a amount        greater than 0.05, in particular greater than 0.10, more in        particular greater than 0.15 parts by weight per 100 parts by        weight of acrylate base polymer;    -   c) a co-polymerized hydrogen-donating monomer; and    -   d) a tackifying resin, in an amount greater than 40 parts by        weight per 100 parts by weight of acrylate base polymer.

In the context of the present invention, the term “adhesive” as usedherein refers to polymeric compositions useful to adhere together twoadherends. Examples of adhesives are pressure sensitive adhesives.

Pressure sensitive adhesive compositions are well known to those ofordinary skill in the art to possess properties including the following:(1) aggressive and permanent tack, (2) adherence with no more thanfinger pressure, (3) sufficient ability to hold onto an adherend, and(4) sufficient cohesive strength. Materials that have been found tofunction well as pressure sensitive adhesives are polymers designed andformulated to exhibit the requisite viscoelastic properties resulting ina desired balance of tack, peel adhesion, and shear holding power.

In the context of the present invention, the term “acrylate” refers tomonomeric acrylic or methacrylic esters of alcohols. Acrylate andmethacrylate monomers, oligomers, or polymers are referred tocollectively herein as “acrylates”.

As will be apparent to those skilled in the art, the term “type (II)photocrosslinker”, used throughout the present description, is meant torefer to a photocrosslinker, which upon irradiation, becomes excited toa higher energy state in which it can abstract a hydrogen atom from ahydrogen-donating molecule, thereby generating on the hydrogen-donatingmolecule a free radical capable of further reaction, such as e.g. freeradical addition polymerization, free radical addition crosslinking. Theexpression “co-polymerized type (II) photocrosslinker” is meant toreflect that the photocrosslinker is co-polymerized with any suitablepolymeric matrix, including the acrylate base polymer and thecrosslinking polymer, as defined hereinbelow.

In the context of the present invention still, the term“hydrogen-donating monomer” used throughout the description, is meant todesignate a monomer which is capable of (relatively easily) donating ahydrogen atom (abstractable hydrogen atom) to a photocrosslinker which,upon irradiation, is excited to a higher energy state which makes itcapable of abstracting an abstractable hydrogen atom, from e.g. ahydrogen-donating monomer. The expression “co-polymerizedhydrogen-donating monomer” is meant to reflect that thehydrogen-donating monomer is co-polymerized with any suitable polymericmatrix, including the acrylate base polymer and the crosslinkingpolymer, as defined hereinbelow.

The term “alkyl” refers to a monovalent group that is a radical of analkane, which is a saturated hydrocarbon. The alkyl can be linear,branched, cyclic, or combinations thereof and typically has 1 to 20carbon atoms. In some embodiments, the alkyl group contains 1 to 18, 1to 12, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms. Examples ofalkyl groups include, but are not limited to, methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, cyclohexyl,n-heptyl, n-octyl, and 2-ethylhexyl.

The term “aryl” refers to a monovalent group that is aromatic andcarbocyclic. The aryl can have one to five rings that are connected toor fused to the aromatic ring. The other ring structures can bearomatic, non-aromatic, or combinations thereof. Examples of aryl groupsinclude, but are not limited to, phenyl, biphenyl, terphenyl, anthryl,naphthyl, acenaphthyl, anthraquinonyl, phenanthryl, anthracenyl,pyrenyl, perylenyl, and fluorenyl.

The terms “glass transition temperature” and “Tg” are usedinterchangeably and refer to the glass transition temperature of amaterial or a mixture. Unless otherwise indicated, glass transitiontemperature values are determined by Differential Scanning Calorimetry(DSC).

The radiation crosslinkable pressure sensitive adhesive precursoraccording to a first aspect of the present invention, comprises anacrylate base polymer. Suitable acrylate base polymer for use hereinwill be easily identified by those skilled in the art, in the light ofthe present description.

In a preferred aspect, the acrylate base polymer for use in the presentinvention, is prepared from a polymerizable mixture comprising at leastone linear or branched alkyl (meth)acrylate monomer, wherein the linearor branched alkyl group of the alkyl (meth)acrylate monomer preferablycomprises from 1 to 24, more preferably from 4 to 20, even morepreferably 6 to 15, still more preferably from 6 to 10 carbon atoms.

In a preferred aspect, the linear or branched alkyl (meth)acrylatemonomer is selected from the group consisting of methyl acrylate, ethylacrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate,isobutyl acrylate, n-pentyl acrylate, iso-pentyl acrylate, n-hexylacrylate, iso-hexyl acrylate, cyclohexyl acrylate, octyl acrylate,iso-octyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, laurylacrylate, 2-propylheptyl acrylate, stearyl acrylate, isobornyl acrylate,octadecyl acrylate, nonyl acrylate, dodecyl acrylate, and anycombinations or mixtures thereof. Also useful are aromatic acrylates,acrylates containing aryl groups, e.g., benzyl acrylate and cyclobenzylacrylate.

More preferably, the alkyl (meth)acrylate monomer for use herein isselected from the group consisting of iso-octyl acrylate, 2-ethylhexylacrylate, butyl acrylate, and any combinations or mixtures thereof.Still more preferably, the alkyl (meth)acrylate monomer for use hereincomprises (or consists of) iso-octyl acrylate.

Typically, the acrylate base polymer for use in the present invention isprepared from a monomer mixture comprising from 50 to 100 parts, from 70to 100 parts, from 80 to 100 parts, or even from 90 to 100 parts byweight of at least one linear or branched alkyl (meth)acrylate monomer,wherein the linear or branched alkyl group of the alkyl (meth)acrylatemonomer preferably comprises from 1 to 24, more preferably from 4 to 20,even more preferably 6 to 15, still more preferably from 6 to 10 carbonatoms.

Optionally, one or more monoethylenically unsaturated co-monomers may bepresent in the (pre-polymerization) monomer mixture used to prepare theacrylate base polymer, in an amount of from 0.5 to 50 parts co-monomer,and are thus typically polymerized with the acrylate monomers. One classof useful co-monomers includes those having a homopolymer glasstransition temperature greater than the glass transition temperature ofthe acrylate homopolymer. Sometimes these monomers are referred to as“reinforcing co-monomers”. Typically these monomers have a homopolymerglass transition temperature greater than 20° C. Examples of suitableco-monomers falling within this class include acrylic acid, itaconicacid, methacrylic acid, acrylonitrile, methacrylonitrile, vinyl acetate,isobornyl acrylate, cyano ethyl acrylate, maleic anhydride,hydroxyalkylacrylates, beta-carboxyethyl acrylate, vinyl esters ofneodecanoic, neononanoic, neopentanoic, 2-ethylhexanoic, or propionicacids (e.g., available from Union Carbide Corp. of Danbury, Conn. underthe designation “Vynates”), vinylidene chloride, styrene, vinyl toluene,alkyl vinyl ethers, and any combinations or mixtures thereof. Preferredco-monomer for use herein includes, but is not limited to, acrylic acid.When present, the monoethylenically unsaturated co-monomer is typicallyused in amounts ranging from 0.5 to 25, from 1.0 to 15, from 1.0 to 8.0,from 2.0 to 6.0, or even from 3.0 to 5.0 parts, by weight per 100 partsby weight of acrylate base polymer.

A second class of useful co-monomers includes those having a homopolymerglass transition temperature less than the glass transition temperatureof the acrylate homopolymer. Examples of suitable co-monomers fallingwithin this class include ethoxyethoxy ethyl acrylate (Tg=−71° C.) andmethoxypolyethylene glycol 400 acrylate (Tg=−65° C.; available from ShinNakamura Chemical Co., Ltd. under the designation “NK Ester AM-90G”).

Accordingly, in some preferred aspects of the radiation crosslinkablepressure sensitive adhesive precursor of the invention, the acrylatebase polymer (or the pre-polymerization monomer mixture used to preparethe acrylate base polymer) comprises a co-polymer of at least one(meth)acrylate monomer, preferably an alkyl (meth)acrylate monomer.

In a preferred aspect, the acrylate base polymer comprises a co-polymerof iso-octyl acrylate, 2-ethylhexyl acrylate, or butyl acrylate andacrylic acid.

In some other preferred aspects, the acrylate base polymer (or thepre-polymerization monomer mixture used to prepare the acrylate basepolymer) further comprises a photocrosslinker, preferably aco-polymerized type (II) photocrosslinker, as defined hereinbelow.

Additionally, one or more multifunctional ethylenically unsaturatedmonomers may be included in the pre-polymerization monomer mixture usedto prepare the acrylate base polymer, typically in low concentration.Examples of such multifunctional ethylenically unsaturated monomersinclude, for example, multifunctional (meth)acrylate monomers.Multifunctional (meth)acrylates include tri(meth)acrylates anddi(meth)acrylates (that is, compounds comprising three or two(meth)acrylate groups). Typically di(meth)acrylate monomers (that is,compounds comprising two (meth)acrylate groups) are used. Usefultri(meth)acrylates include, for example, trimethylolpropanetri(meth)acrylate, propoxylated trimethylolpropane triacrylates,ethoxylated trimethylolpropane triacrylates, tris(2-hydroxyethyl)isocyanurate triacrylate, and pentaerythritol triacrylate. Usefuldi(meth)acrylates include, for example, ethylene glycoldi(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycoldi(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,4-butanedioldi(meth)acrylate, 1,6-hexanediol di(meth)acrylate, alkoxylated1,6-hexanediol diacrylate, tripropylene glycol diacrylate, dipropyleneglycol diacrylate, cyclohexane dimethanol di(meth)acrylate, alkoxylatedcyclohexane dimethanol diacrylates, ethoxylated bisphenol Adi(meth)acrylates, neopentyl glycol diacrylate, polyethylene glycoldi(meth)acrylates, polypropylene glycol di(meth)acrylates, and urethanedi(meth)acrylates. The branching agent 1,6-hexanediol diacrylate (HDDA)is particularly suitable. Typically the di(meth)acrylate branching agentis used in amounts ranging from 0.001 to 0.05 parts by weight per 100parts by weight of acrylate base polymer.

Generally, the pre-polymerization monomer mixture used to prepare theacrylate base polymer, includes an appropriate initiator. Forpolymerization by ultraviolet light, a photoinitiator is included.Useful photoinitiators include substituted acetophenones such as benzyldimethyl ketal and 1-hydroxycyclohexyl phenyl ketone, substitutedalpha-ketols such as 2-methyl-2-hydroxypropiophenone, benzoin etherssuch as benzoin methyl ether, benzoin isopropyl ether, substitutedbenzoin ethers such as anisoin methyl ether, aromatic sulfonylchlorides, photoactive oximes and azo-type initiators. Thephotoinitiator may be used in an amount from about 0.001 to about 5.0parts by weight, preferably from about 0.01 to about 5.0 parts byweight, more preferably in an amount from 0.05 to 0.5 parts by weight,and more preferably in an amount from 0.05 to 0.3 parts by weight per100 parts by weight of total monomer.

The pre-polymerization monomer mixture used to prepare the acrylate basepolymer, may also be polymerized by thermal polymerization or by acombination of thermal and radiation polymerization. For thermalpolymerization, a thermal initiator is included. Thermal initiatorsuseful in the present invention include, but are not limited to azo,peroxide, persulfate, and redox initiators. Azo-type initiators, such ase.g. the “Vazo” line, commercially available from DuPont Chemical Co,are particularly preferred. The thermal initiator may be used in anamount from about 0.01 to about 5.0 parts by weight per 100 parts byweight of total monomer, preferably from 0.025 to 2 weight percent.

The radiation crosslinkable pressure sensitive adhesive precursoraccording to a first aspect of the present invention, further comprisesa co-polymerized type (II) photocrosslinker. Suitable co-polymerizedtype (II) photocrosslinkers for use herein will be easily identified bythose skilled in the art, in the light of the present description.

In some exemplary aspects, the co-polymerized type (II)photocrosslinkers for use in the present invention are selected from thegroup consisting of mono-and multi-ethylenically unsaturated aromaticketone comonomers free of ortho-aromatic hydroxyl groups such as thosedisclosed in U.S. Pat. No. 4,737,559 (Kellen et al.). Specific examplesof mono-ethylenically unsaturated aromatic ketone comonomers include thecopolymerizable photosensitive crosslinkers para-acryloxybenzophenone(ABP), para-acryloxyethoxybenzophenone (AEBP),para-N-(methylacryloxyethyl)-carbamoylethoxybenzophenone,4-acryloyloxydiethoxy-4-chlorobenzophenone, para-acryloxyacetophenone,ortho-acrylamidoacetophenone, acrylated anthraquinones, and anycombinations or mixtures thereof.

In a preferred aspect, the co-polymerized type (II) photocrosslinker foruse in the present invention is selected from the group ofmono-ethylenically unsaturated aromatic ketones. More preferably, theco-polymerized type (II) photocrosslinker for use in the presentinvention is selected from the group consisting ofpara-acryloxybenzophenone (ABP), para-acryloxyethoxybenzophenone (AEBP),para-N-(methylacryloxyethyl)-carbamoylethoxybenzophenone,4-acryloyloxydiethoxy-4-chlorobenzophenone, para-acryloxyacetophenone,ortho-acrylamidoacetophenone, acrylated anthraquinones, and anycombinations or mixtures thereof. Even more preferably, theco-polymerized type (II) photocrosslinker for use in the presentinvention is selected from the group consisting ofpara-acryloxybenzophenone (ABP), para-acryloxyethoxybenzophenone (AEBP),and any combinations or mixtures thereof.

The co-polymerized type (II) photocrosslinkers may typically be used inan amount from 0.06 to 1 parts, from 0.11 to 1 parts, from 0.16 to 1parts, from 0.18 to 0.70 parts, or even from 0.20 to 0.50 parts byweight per 100 parts by weight of acrylate base polymer (or ofpre-polymerization monomer mixture used to prepare the acrylate basepolymer).

In one exemplary aspect of the radiation crosslinkable pressuresensitive adhesive precursor of the invention, the co-polymerized type(II) photocrosslinker for use herein may be co-polymerized into theacrylate base polymer, i.e. co-polymerized together with the othermonomers present into the pre-polymerization monomer mixture used toprepare the acrylate base polymer.

In an alternative exemplary aspect of the radiation crosslinkablepressure sensitive adhesive precursor of the invention, theco-polymerized type (II) photocrosslinker for use herein may beco-polymerized into a crosslinking polymer, preferably an acrylatecrosslinking polymer, as detailed herein below, and distinct from theacrylate base polymer, i.e. co-polymerized together with the othermonomers present into the pre-polymerization monomer mixture used toprepare the crosslinking polymer.

The radiation crosslinkable pressure sensitive adhesive precursoraccording to a first aspect of the present invention, further comprisesa co-polymerized hydrogen-donating monomer. Suitable co-polymerizedhydrogen-donating monomers for use herein will be easily identified bythose skilled in the art, in the light of the present description.

Exemplary co-polymerized hydrogen-donating monomers include, but are notlimited to, monomers comprising at least one abstractable hydrogen atomtypically located on a carbon atom in a position alpha to a nitrogen oran oxygen atom, or carried by terminal or pendant mercapto groupspotentially protected during polymerization. According to one preferredaspect, the co-polymerized hydrogen-donating monomer for use herein isselected from the group of monomers comprising at least one hydrogenatom on a carbon atom in a α-position to a nitrogen atom.

In some aspects, the co-polymerized hydrogen-donating monomer for useherein is selected from the group consisting of (meth)acrylamide,(meth)acrylic, urethane (meth)acrylic, styrenic and vinylic monomerscontaining at least one amine functional group, preferably a tertiaryamine functional group, and any combinations or mixtures thereof.

In another particular aspect of the radiation crosslinkable pressuresensitive adhesive precursor of the invention, the co-polymerizedhydrogen-donating monomer is selected from the group consisting ofN,N-dimethyl acrylamide; N-vinyl caprolactam; N-Vinylpyrrolidone;N-isopropyl acrylamide; N,N-dimethylaminoethyl methacrylate;2-[[(Butylamino)carbonyl]oxy]ethyl acrylate N,N-dimethylaminopropylmethacrylamide; N,N-diethylaminopropyl methacrylamide;N,N-dimethylaminoethyl acrylate; N,N-diethylaminoethyl acrylate;N,N-dimethylaminopropyl acrylate; N,N-diethylaminopropyl acrylate;N,N-diethylaminoethyl methacrylate; N,N-dimethylaminoethyl acrylamide;N,N-dimethylaminoethyl methacrylamide; N,N-diethylaminoethyl acrylamide;N,N-diethylaminoethyl methacrylamide; and any combinations or mixturesthereof. Preferably still, the co-polymerized hydrogen-donating monomerfor use herein is selected from the group consisting of N,N-dimethylacrylamide; N-vinyl caprolactam; N-isopropyl acrylamide;N,N-dimethylaminoethyl methacrylate; 2-[[(Butylamino)carbonyl]oxy]ethylacrylate and any combinations or mixtures thereof.

The co-polymerized hydrogen-donating monomer may typically be used in anamount from 0.05 to 10 parts, from 0.05 to 5 parts, from 0.10 to 3parts, or even from 0.15 to 2 parts by weight per 100 parts by weight ofacrylate base polymer (or of pre-polymerization monomer mixture used toprepare the acrylate base polymer).

In one exemplary aspect of the radiation crosslinkable pressuresensitive adhesive precursor of the invention, the co-polymerizedhydrogen-donating monomer for use herein may be co-polymerized with theacrylate base polymer, i.e. co-polymerized together with the othermonomers present into the pre-polymerization monomer mixture used toprepare the acrylate base polymer.

In an alternative exemplary aspect of the radiation crosslinkablepressure sensitive adhesive precursor of the invention, theco-polymerized hydrogen-donating monomer for use herein may beco-polymerized with a crosslinking polymer, preferably an acrylatecrosslinking polymer, i.e. co-polymerized together with the othermonomers present into the pre-polymerization monomer mixture used toprepare the crosslinking polymer, as detailed herein below, and whereinthe crosslinking polymer is distinct from the acrylate base polymer.

The radiation crosslinkable pressure sensitive adhesive precursoraccording to the present invention, further comprises a tackifyingresin. Suitable tackifying resin for use herein will be easilyidentified by those skilled in the art, in the light of the presentdescription.

The tackifying resin or resins are added to the radiation crosslinkablepressure sensitive adhesive precursor at levels to give what are calledin this disclosure a “highly tackified pressure sensitive adhesive”(typically greater than 40 parts by weight tackifying resin per 100parts by weight of acrylate base polymer).

Typically, (meth)acrylate based adhesives require little or notackifying resins to achieve desired pressure sensitive adhesiveproperties. The use of high levels of tackifying agent(s) may bedesirable because it can increase the tackiness of the pressuresensitive adhesive, making it aggressively adhere to wide range ofsubstrates without the need to apply pressure. The addition oftackifying resin, especially high levels of tackifying resin, candetrimentally affect the shear and cohesive strength of a pressuresensitive adhesive, and can raise the Tg of the adhesive. The use ofhigh levels of tackifying resin can be particularly detrimental to hotmelt processable pressure sensitive adhesives where the need to be hotmelt processable can already adversely affect the shear strength andcohesive strength properties of the adhesive. However, the adhesives ofthe present disclosure comprise greater than 40 parts by weight oftackifying resin per 100 parts by weight of acrylate base polymer. Thisrelatively high level of tackifying resin is achieved withoutsignificant negative effects on the shear properties of the adhesive. Insome particular aspects, the amount of tackifying resin present in theradiation crosslinkable pressure sensitive adhesive precursor is greaterthan 45 parts, greater than 50 parts, greater than 55 parts, or evengreater than 60 parts by weight per 100 parts by weight of acrylate basepolymer. In some other aspects, the radiation crosslinkable pressuresensitive adhesive precursor comprises from 45 to 85 parts, from 45 to75 parts, from 45 to 65 parts, or even from 50 to 60 parts by weight oftackifying resin per 100 parts by weight of acrylate base polymer.

Suitable tackifying resins include, for example, terpene phenolics,rosins, rosin esters, esters of hydrogenated rosins, synthetichydrocarbon resins and combinations thereof. Especially suitabletackifying resins include the commercially available tackifying resins:FORAL 85E (a glycerol ester of highly hydrogenated refined gum rosin)commercially available from Eastman, Middelburg, NL), FORAL 3085 (aglycerol ester of highly hydrogenated refined wood rosin) commerciallyavailable from Hercules Inc., Wilmington, Del.; ESCOREZ 2520 and ESCOREZ5615 (aliphatic/aromatic hydrocarbon resins) commercially available fromExxonMobil Corp., Houston, Tex.; and Regalite 7100 (a partiallyhydrogenated hydrocarbon resin) commercially available from Eastman,Kingsport, Tenn.

In some aspects, it may be desirable to use a mixture of two tackifyingresins, where one of the tackifying resins comprises a high Tgtackifying resin with a glass transition temperature of at least 20° C.,and the other comprises a low Tg tackifying resin with a glasstransition temperature of no greater than 0° C. Such mixtures oftackifying resins are described, for example, in PCT Patent PublicationNo. WO 2010/002557 (Ma et al.). The high Tg tackifying resin istypically a solid at room temperature. Examples of suitable high Tgtackifying resin include, for example, terpenes, aliphatic- oraromatic-modified C5 to C9 hydrocarbons, and rosin esters. In someaspects, lower molecular weight hydrocarbons may be preferred, ascompatibility with the acrylate base polymer decreases as the molecularweight of the hydrocarbon increases. In some aspects, the weight averagemolecular weight (Mw) of the high Tg tackifying resin is between 500 and3000 gm/mole. In some aspects, the Mw of the high Tg tackifying resin isno greater than 1500, in some embodiments no greater than 1000, or evenno greater than 800 gm/mole.

The low Tg tackifying resin has a glass transition temperature of nogreater than 0° C., in some aspects, no greater than −10° C., or even nogreater than −20° C. Such materials are generally liquids at roomtemperature. There is no particular lower limit on the glass transitiontemperature of the low Tg tackifying resin, except that it must begreater than the Tg the acrylate base polymer. In some aspects, the Tgof the low Tg tackifying resin is at least 10° C. greater, at least 20°C. greater, or even at least 30° C. greater than the Tg of the acrylatebase polymer. Generally, lower molecular weight compounds may be moredesirable, as compatibility with the acrylate base polymer decreases asthe molecular weight of the increases. Exemplary low Tg tackifyingresins include terpene phenolic resins, terpenes, aliphatic- oraromatic-modified C5 to C9 hydrocarbons, and rosin esters. In someaspects, the weight average molecular weight (Mw) of the low Tgtackifying resin is between 300 and 1500 gm/mole. In some aspects, theMw of the low Tg tackifying resin is no greater than 1000, in someaspects, no greater than 800, or even no greater than 500 gm/mole.

In some aspects, the adhesive precursors comprise from 35 to 65 parts byweight of the high Tg tackifying resin per 100 parts by weight ofacrylate base polymer. In some aspects, the adhesive precursors compriseat least 40 parts by weight of the high Tg tackifying resin per 100parts by weight of acrylate base polymer. In some aspects, the adhesiveprecursors comprise greater than 50 parts by weight or even at least 60parts by weight of the high Tg tackifying resin per 100 parts by weightof acrylate base polymer.

In some aspects, the adhesive precursors comprise from 2 to 20 parts byweight of low Tg tackifying resin per 100 parts by weight of acrylatebase polymer. In some aspects, the adhesives comprise at least from 5 to18, or even from 5 to 17 parts by weight of low Tg tackifying resin per100 parts by weight of acrylate base polymer.

A wide variety of commercially available tackifying resins are availableand are suitable for use as the high Tg tackifying resin and the low Tgtackifying resin. Especially suitable High Tg tackifying resins includethe commercially available tackifying resins: FORAL 3085 and FORAL 85LBresins commercially available from Hercules Inc., Wilmington, Del.; andSP-553 from Schenectady International, Schenectady, N.Y., with FORAL3085 being especially desirable. Especially suitable Low Tg tackifyingresins include the commercially available tackifying resins: ESCOREZ2520 commercially available from ExxonMobil Corp., Houston, Tex.,STAYBELITE Ester 3-E commercially available from Eastman Chemical,Kingsport, Tenn., PICCOLYTE AO commercially available from Hercules,Inc., Wilimington, Del., and HERCOLYN D commercially available fromHercules, Inc., Wilimington, Del., with ESCOREZ 2520 being especiallydesirable.

In a preferred aspect of the radiation crosslinkable pressure sensitiveadhesive precursor according to the invention, the tackifying resin isselected from the group consisting of terpene phenolics, rosins, rosinesters, esters of hydrogenated rosins, synthetic hydrocarbon resins, andany combinations or mixtures thereof. More preferably, the tackifyingresin is selected from the group of esters of hydrogenated rosins.

In some aspects of the present invention, the radiation crosslinkablepressure sensitive adhesive precursor according to the present inventionmay optionally comprise a crosslinking polymer.

Suitable compositions for forming a crosslinking polymer for use hereinwill be easily identified by those skilled in the art, in the light ofthe present disclosure. Exemplary compositions useful for preparing acrosslinking polymer for use herein include, but are not limited to,those comprising a monomer mixture comprising monomers selected from thegroup consisting of acrylic monomers, vinyl ester monomers, acryl amidemonomers, alkyl (meth)acryl amide monomers, dialkyl acryl amide,styrenic monomers, and any combinations or mixtures thereof.

Accordingly, crosslinking polymers for use herein may be acrylate, vinylester, acryl amide, alkyl acryl amide, dialkyl acryl amide or styrene(co)polymers, including in particular monomers such as e. g. alkyl(meth)acrylamide monomers, di aryl (meth)acrylamide monomers, styrenicmonomers (in particular low Tg styrenic monomers such as e.g.butoxy-styrene monomers), vinyl ester monomers, and any combinations ormixtures thereof.

In a preferred aspect, the crosslinking polymer is an acrylatecrosslinking polymer. Compositions useful for forming the acrylatecrosslinking polymer for use herein may be identical or distinct fromthe compositions used for forming the acrylate base polymer, asdescribed herein above.

In a preferred aspect, the acrylate crosslinking polymer for use in thepresent invention, is prepared from a monomer mixture comprising atleast one linear or branched alkyl (meth)acrylate monomer, wherein thelinear or branched alkyl group of the alkyl (meth)acrylate monomerpreferably comprises from 1 to 24, more preferably from 4 to 20, evenmore preferably 6 to 15, still more preferably from 6 to 10 carbonatoms.

In still a preferred aspect, the linear or branched alkyl (meth)acrylatemonomer is selected from the group consisting of methyl acrylate, ethylacrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate,isobutyl acrylate, n-pentyl acrylate, iso-pentyl acrylate, n-hexylacrylate, iso-hexyl acrylate, cyclohexyl acrylate, octyl acrylate,iso-octyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, laurylacrylate, 2-propylheptyl acrylate, stearyl acrylate, isobornyl acrylate,and any combinations or mixtures thereof.

More preferably, the alkyl (meth)acrylate monomer for use herein isselected from the group consisting of iso-octyl acrylate, 2-ethylhexylacrylate, butyl acrylate, and any combinations or mixtures thereof.Still more preferably, the alkyl (meth)acrylate monomer for use hereincomprises (or consists of) iso-octyl acrylate.

According to a particular aspect, a vinyl ester (co)-monomer, preferablya vinyl ester of versatic acid (co)-monomer, may be present in the(pre-polymerization) monomer mixture used to prepare the crosslinkingpolymer, typically in an amount of from 0 to 50 parts co-monomer, and isthus typically (co)polymerized with the acrylate monomers. Suitablevinyl ester of versatic acid (co)-monomers include the commerciallyavailable monomer: Veova 10, commercially available from Momentive,Columbus, Ohio. Typically the vinyl ester co-monomer is used in amountsranging from 0.5 to 40 parts, from 1.0 to 30 parts, from 5 to 25 parts,from 10 to 20 parts, or even from 15 to 20 parts, by weight per 100parts by weight of acrylate crosslinking polymer.

In some aspects, the radiation crosslinkable pressure sensitive adhesiveprecursor mixture comprises from 0.5 to 30 parts, from 0.5 to 20 parts,from 1.0 to 10 parts, or even from 2.0 to 8.0 parts by weight per 100parts by weight of acrylate base polymer, of the crosslinking polymer,preferably the acrylate crosslinking polymer.

In a particular aspect of the radiation crosslinkable pressure sensitiveadhesive precursor of the invention, the co-polymerized type (II)photocrosslinker and/or the co-polymerized hydrogen-donating monomer foruse herein may be co-polymerized with the crosslinking polymer,preferably the acrylate crosslinking polymer, i.e. co-polymerizedtogether with the other monomers present into the pre-polymerizationmonomer mixture used to prepare the crosslinking polymer, preferably theacrylate crosslinking polymer.

Generally, the pre-polymerization monomer mixture used to prepare thecrosslinking polymer, includes an appropriate initiator.

For thermal polymerization, a thermal initiator is included. Thermalinitiators useful in the present invention include, but are not limitedto azo, peroxide, persulfate, and redox initiators. Azo-type initiators,such as e.g. the “Vazo” line, commercially available from DuPont

Chemical Co, are particularly preferred. The thermal initiator may beused in an amount from about 0.01 to about 5.0 parts by weight per 100parts by weight of total monomer, preferably from 0.025 to 2 weightpercent.

For polymerization by ultraviolet light, a photoinitiator is included.Useful photoinitiators include those described herein above for use inthe acrylate base polymer. The photoinitiator may be used in an amountfrom about 0.001 to about 5.0 parts by weight per 100 parts of totalmonomer, from about 0.01 to about 5.0 parts by weight per 100 parts byweight of total monomer, or even from 0.1 to 0.5 parts by weight per 100parts by weight of total monomer.

Typically, the radiation crosslinkable pressure sensitive adhesiveprecursor mixture according to the invention may further include, as anoptional ingredient, a chain transfer agent to control the molecularweight of the polymer. Advantageously, the chain transfer agent iscomprised in the (pre-polymerization) monomer mixture used to preparethe acrylate base polymer and/or the crosslinking polymer. Chaintransfer agents are materials which regulate free radical polymerizationand are generally known in the art. Suitable chain transfer agentsinclude halogenated hydrocarbons such as carbon tetrabromide; sulfurcompounds such as lauryl mercaptan, butyl mercaptan, ethanethiol,isooctylthioglycolate (IOTG), 2-ethylhexyl thioglycolate, 2-ethylhexylmercaptopropionate, pentaerythritol terakis(3-mercaptopropionate),2-mercaptoimidazole, and 2-mercaptoethyl ether; and solvents such asethanol and isopropanol.

As will be apparent to those skilled in the art, the radiationcrosslinkable pressure sensitive adhesive precursor mixture according tothe invention may further include a variety of additional additivesdepending on the envisaged properties for the resulting crosslinkedpressure sensitive adhesive. Exemplary additional additives include, butare not limited to, one or more plasticizers, UV stabilizers, antistaticagents, colorants, antioxidants, fungicides, bactericides, organicand/or inorganic filler particles, pigments, and any combinationsthereof. Advantageously, the additional additives for use herein arenon-polymerizable additives. As will be apparent to those skilled in theart, additional additives for use herein may be included at appropriatetiming and in the appropriate polymeric or pre-polymeric matrix.

In an advantageous aspect of the radiation crosslinkable pressuresensitive adhesive precursor according to the invention, the amount ofacrylate base polymer and co-polymerized hydrogen-donating monomer areselected such as to provide the radiation crosslinked pressure sensitiveadhesive obtained by radiation crosslinking, preferably UV radiationcrosslinking, the radiation crosslinkable pressure sensitive adhesiveprecursor, with a static shear at 70° C. of at least 2000 minutes,preferably at least 4000 minutes, more preferably at least 6000 minutes,even more preferably at least 8000 minutes, still more preferably atleast 10000 minutes, when measured according to static shear test ASTMD3654.

In an advantageous aspect, the static shear at 70° C. is measured on aUV crosslinked pressure sensitive adhesive layer coated on a substrate,wherein the thickness of the pressure sensitive adhesive layer istypically of about 100 μm, and wherein the UV crosslinking is typicallyoperated with 800 mJ/cm² UV from a medium pressure UV lamp,corresponding to 250 mJ/cm² UV-B light and 50 mJ/cm² UV-C.

According to one preferred aspect of the invention, the radiationcrosslinkable pressure sensitive adhesive precursor comprises:

-   -   a) an acrylate base polymer;    -   b) from 0.06 to 1 parts, from 0.11 to 1 parts, from 0.16 to 1        parts, from 0.18 to 0.70 parts, or even from 0.20 to 0.50 parts        by weight per 100 parts by weight of acrylate base polymer, of a        co-polymerized type (II) photocrosslinker;    -   c) from 0.05 to 30 parts, from 0.05 to 10 parts, from 0.05 to 5        parts, from 0.10 to 3 parts, or even from 0.15 to 2 parts by        weight per 100 parts by weight of acrylate base polymer, of a        co-polymerized hydrogen-donating monomer; and    -   d) from 40 to 80 parts, from 40 to 60 parts, or even from 45 to        55 parts by weight per 100 parts by weight of acrylate base        polymer, of a tackifying resin.

In an advantageous aspect of the invention, the radiation crosslinkablepressure sensitive adhesive precursor is hot melt processable. But theinvention is not that limited, as according to another advantageousaspect, the radiation crosslinkable pressure sensitive adhesiveprecursor may be provided as a solvent borne adhesive system, which istherefore solvent processable, or as a water based system.

Hot melt processable radiation crosslinkable pressure sensitive adhesiveprecursors for use herein are typically hot melt mixed blends comprisingan acrylate base polymer, a co-polymerized type (II) photocrosslinker,in a amount greater than 0.15 parts by weight per 100 parts by weight ofacrylate base polymer, a co-polymerized hydrogen-donating monomer, and atackifying resin, in an amount greater than 40 parts by weight per 100parts by weight of acrylate base polymer. Typically, the hot meltprocessable radiation crosslinkable pressure sensitive adhesiveprecursor may further comprise a thermoplastic material.

The hot melt processable radiation crosslinkable pressure sensitiveadhesive precursors for use herein may be prepared by a variety of hotmelt techniques. Generally, the methods comprise providing a hot meltmixing apparatus, providing an acrylate base polymer, a co-polymerizedtype (II) photocrosslinker in a amount greater than 0.15 parts by weightper 100 parts by weight of acrylate base polymer, a co-polymerizedhydrogen-donating monomer, and providing greater than 40 parts by weightper 100 parts by weight of acrylate base polymer of a tackifying resinin an amount, mixing the acrylate base polymer, the co-polymerized type(II) photocrosslinker, the co-polymerized hydrogen-donating monomer andthe tackifying resin in the hot melt mixing apparatus to prepare a hotmelt blend, removing the blend from the hot melt mixing apparatus toform a hot melt processable pressure sensitive adhesive.

As described below, a variety of additional additives can be included inthe hot melt blend including one or more plasticizers, crosslinkers, UVstabilizers, antistatic agents, colorants, antioxidants, fungicides,bactericides, organic and/or inorganic filler particles, and the like.Optionally, low levels of plasticizer (e.g., less than about 10 parts byweight) may be added to the hot melt blend. A wide variety ofcommercially available materials described as “plasticizers” aresuitable, as long as the added plasticizer is compatible with the othercomponents of the hot melt blend. Representative plasticizers includepolyoxyethylene aryl ether, dialkyl adipate, 2-ethylhexyl diphenylphosphate, t-butylphenyl diphenyl phosphate, di(2-ethylhexyl) adipate,toluenesulfonamide, dipropylene glycol dibenzoate, polyethylene glycoldibenzoate, polyoxypropylene aryl ether, dibutoxyethoxyethyl formal, anddibutoxyethoxyethyl adipate. Especially suitable is the plasticizerSANTICIZER 141 (2-ethylhexyl diphenyl phosphate) commercially availablefrom Ferro Corp., Cleveland, Ohio.

A variety of hot melt mixing techniques using a variety of hot meltmixing equipment are suitable for preparing the hot melt processablepressure sensitive adhesive precursors and hot melt processable pressuresensitive adhesives. Both batch and continuous mixing equipment may beused. Examples of batch methods include those using a BRABENDER (e. g. aBRABENDER PREP CENTER, commercially available from C.W. BrabenderInstruments, Inc.; South Hackensack, N.J.) or BANBURY internal mixingand roll milling equipment (e.g. equipment available from Farrel Co.;Ansonia, Conn.). Examples of continuous methods include single screwextruding, twin screw extruding, disk extruding, reciprocating singlescrew extruding, pin barrel single screw extruding, planetary extruding,and ring extruding. Continuous methods can utilize distributiveelements, pin mixing elements, static mixing elements, and dispersiveelements such as MADDOCK mixing elements and SAXTON mixing elements. Asingle hot melt mixing apparatus may be used, or a combination of hotmelt mixing equipment may be used to prepare the hot melt blends and thehot melt processable pressure sensitive adhesives. In some embodiments,it may be desirable to use more than one piece of hot melt mixingequipment. For example, one extruder, such as, for example, a singlescrew extruder, can be used to hot melt process the hot melt processableelastomeric (meth)acrylate random copolymer contained within athermoplastic pouch. The output of this extruder can be fed into asecond extruder, for example, a twin screw extruder for hot melt mixingwith the additional components. The hot melt blends described above areused to form hot melt processable pressure sensitive adhesives uponcompletion of the hot melt blending process.

The output of the hot melt mixing is coated onto a substrate to form anadhesive layer. If a batch apparatus is used, the hot melt blend can beremoved from the apparatus and placed in a hot melt coater or extruderand coated onto a substrate. If an extruder is used to prepare the hotmelt blend, the blend can be directly extruded onto a substrate to forman adhesive layer in a continuous forming method. In the continuousforming method, the adhesive can be drawn out of a film die andsubsequently contacted to a moving plastic web or other suitablesubstrate. If the adhesive is to be part of a tape, the substrate may bea tape backing. In some methods, the tape backing material is coextrudedwith the adhesive from a film die and the multilayer construction isthen cooled to form the tape in a single coating step. If the adhesiveis to be a transfer tape, the adhesive layer may be a free standing filmand the substrate may be a release liner or other releasing substrate.After forming, the adhesive layer or film can be solidified by quenchingusing both direct methods (e.g. chill rolls or water batch) and indirectmethods (e.g. air or gas impingement).

In alternative aspects of the invention however, the radiationcrosslinkable pressure sensitive adhesive precursor may be coated usingany other conventional technique well-known in the art, such as e.g.solution coating, emulsion coating, extrusion coating, coextrusioncoating, solventless coating, waterborne coating, and any combinationsthereof.

The resulting radiation crosslinkable pressure sensitive adhesiveprecursor layer is then subjected to a crosslinking step by beingexposed to suitable radiation, in particular to high intensity UVradiation, to effect crosslinking. Typically, the crosslinking step isperformed by exposure to high intensity ultraviolet (UV) radiation usingcommonly known UV lamps with an emission spectra between 250 to 400nanometers (nm), and with an intensity between about 0.1 to about 1000mW/cm².

According to another aspect, the present invention relates to a methodof preparing a radiation crosslinked pressure sensitive adhesive,comprising the steps of:

-   -   a) providing a radiation crosslinkable pressure sensitive        adhesive precursor as above-described; and    -   b) radiation crosslinking the radiation crosslinkable pressure        sensitive adhesive precursor.

The radiation crosslinkable pressure sensitive adhesive precursoraccording to the invention may be obtained using techniques commonlyknown to those skilled in the art of formulating pressure sensitiveadhesive formulations. The polymeric precursor may be obtained in aconventional manner, using e.g. solution, bulk, or emulsionpolymerization techniques. The acrylate base polymer may advantageouslybe obtained using bulk or solution polymerization using thermal or UVtechniques. The crosslinking polymer may advantageously be obtainedusing solution polymerization, followed by stripping of the solventthereby forming a polymelt.

Depending on whether the type (II) photocrosslinker and/or thehydrogen-donating monomer are co-polymerized with the acrylate basepolymer and/or with the crosslinking polymer, the variouspre-polymerizations formulations and the corresponding monomer mixtureswill be easily apparent to those skilled in the art in the light of thepresent description.

Typically, the polymerization steps for the acrylate base polymer areeffected by exposure to ultraviolet (UV) radiation as described in U.S.Pat. No. 4,181,752 (Martens et al.). In some executions, thepolymerization is carried out with UV black lights having over 60percent, or over 75 percent of their emission spectra between 280 to 400nanometers (nm), with an intensity between about 0.1 to about 25 mW/cm2.

The weight average molecular weight of the polymer (i.e. the acrylatebase polymer and/or the crosslinking polymer) having a co-polymerizedtype (II) photocrosslinker may advantageously range from about 50,000 toabout 3,000,000, or from about 100,000 to about 1,800,000, and moretypically from about 200,000 to about 1,500,000.

In one advantageous aspect, the method of preparing a radiationcrosslinked pressure sensitive adhesive, comprises the steps of:

-   -   a) providing a radiation crosslinkable pressure sensitive        adhesive precursor as above-described; and    -   b) radiation crosslinking the radiation crosslinkable pressure        sensitive adhesive precursor.

According to one exemplary aspect, the method of preparing a radiationcrosslinked pressure sensitive adhesive comprises the steps of:

-   -   a) providing an acrylate base polymer;    -   b) providing a co-polymerized type (II) photocrosslinker, in a        amount greater than 0.05 parts, greater than 0.10 parts, or even        greater than 0.15 parts by weight per 100 parts by weight of        acrylate base polymer;    -   c) providing a co-polymerized hydrogen-donating monomer;    -   d) providing a tackifying resin, in an amount greater than 40        parts by weight per 100 parts by weight of acrylate base        polymer;    -   e) mixing the acrylate base polymer, co-polymerized type (II)        photocrosslinker, the co-polymerized hydrogen-donating monomer        and the tackifying resin thereby forming a radiation        crosslinkable pressure sensitive adhesive precursor; and    -   f) radiation crosslinking, preferably UV radiation crosslinking,        the radiation crosslinkable pressure sensitive adhesive        precursor thereby forming a radiation crosslinked pressure        sensitive adhesive.

In one particular aspect of the method of preparing a radiationcrosslinked pressure sensitive adhesive, the co-polymerized type (II)photocrosslinker and/or the co-polymerized hydrogen-donating monomer areco-polymerized with the acrylate base polymer and/or with a crosslinkingpolymer, preferably an acrylate crosslinking polymer, distinct from theacrylate base polymer.

In another particular aspect of the method of preparing a radiationcrosslinked pressure sensitive adhesive, the co-polymerized type (II)photocrosslinker and/or the co-polymerized hydrogen-donating monomer areco-polymerized with the acrylate base polymer.

In still another particular aspect of the method of preparing aradiation crosslinked pressure sensitive adhesive, the co-polymerizedtype (II) photocrosslinker and/or the co-polymerized hydrogen-donatingmonomer are co-polymerized with the crosslinking polymer, preferably theacrylate crosslinking polymer.

In a preferred execution of the method of preparing a radiationcrosslinked pressure sensitive adhesive, the amount of acrylate basepolymer and co-polymerized hydrogen-donating monomer are selected suchas to provide the radiation crosslinked pressure sensitive adhesiveobtained by radiation crosslinking, preferably UV radiationcrosslinking, the radiation crosslinkable pressure sensitive adhesiveprecursor, with a static shear at 70° C. of at least 2000 minutes,preferably at least 4000 minutes, more preferably at least 6000 minutes,even more preferably at least 8000 minutes, still more preferably atleast 10000 minutes, when measured according to static shear test ASTMD3654.

In an advantageous aspect of the method of preparing a radiationcrosslinked pressure sensitive adhesive, the static shear at 70° C. ismeasured on a UV crosslinked pressure sensitive adhesive layer coated ona substrate, wherein the thickness of the pressure sensitive adhesivelayer is typically of about 100 μm, and wherein the UV crosslinking istypically operated with 250 mJ/cm² UV-B light.

Advantageously, the method of preparing a radiation crosslinked pressuresensitive adhesive comprises the step of melt mixing the acrylate basepolymer, the co-polymerized type (II) photocrosslinker, theco-polymerized hydrogen-donating monomer and the tackifying resinthereby forming a hot melt processable radiation crosslinkable pressuresensitive adhesive precursor.

According to another aspect of the present invention, it is provided aradiation crosslinked pressure sensitive adhesive obtainable byradiation crosslinking the radiation crosslinkable pressure sensitiveadhesive precursor precursor as above described.

In another aspect, it is provided a radiation crosslinked pressuresensitive adhesive, as above described, coated on a substrate, whereinthe radiation crosslinked pressure sensitive adhesive coating haspreferably a thickness of at least 40 μm, or greater than 50 μm, greaterthan 70 μm, greater than 80 μm, greater than 90 μm, or even greater than100 μm.

Advantageously, the radiation crosslinked pressure sensitive adhesive asabove described is hot melt processable.

The radiation crosslinked pressure sensitive adhesives and radiationcrosslinked pressure sensitive adhesive precursors of the invention, inparticular the hot melt and solution processable adhesives andprecursors, may advantageously be used to prepare a wide range ofadhesive tapes and articles. Many of these tapes and articles containbackings or other substrates to support the layer of adhesive. Otheradhesive tapes and articles do not contain a backing or substrate layerand therefore are free standing adhesive layers. Double-sided tapes arean example of such an adhesive article. Double-sided tapes, also called“transfer tapes”, are adhesive tapes that have adhesive on both exposedsurfaces. In some transfer tapes, the exposed surfaces are simply thetwo surfaces of a single adhesive layer. Other transfer tapes aremulti-layer transfer tapes with at least two adhesive layers that may bethe same or different, and in some instances intervening layers that maynot be adhesive layers. For example, a multi-layer transfer tape may bea 3 layer construction with an adhesive layer, a film layer and anotheradhesive layer. The film layer can provide handling and/or tear strengthor other desirable properties. In this disclosure, double-sidedadhesives are prepared that comprise one free standing layer of pressuresensitive adhesive.

Since the double-sided adhesives are free standing, they must havesufficient handling strength to be handled without the presence of asupporting layer. However, in many aspects it is desirable that theadhesive layer be readily tearable, that is to say that the adhesivelayer can be readily torn by hand without requiring the use of a cuttingimplement such as a knife, scissors, or a razor blade.

Advantageously, the methods described in this disclosure may be used toform a variety of adhesive articles. Among these adhesive articles aretapes, including transfer tapes. As described above, transfer tapes maybe free standing adhesive films with adhesive on both exposed surfaces.Transfer tapes are widely used in the printing and paper makingindustries for making flying splices, as well being used for a varietyof bonding, mounting, and matting applications both by industry and byconsumers.

Transfer tapes can be prepared in particular by hot melt coating the hotmelt blends described above onto a release surface such as a releaseliner. “Release liners” are well known film articles that have a lowaffinity for adhesives, especially pressure sensitive adhesives. A widevariety of release liners are known and are suitable for use with thepressure sensitive adhesives of this disclosure. Exemplary releaseliners include those prepared from paper (e.g., Kraft paper) orpolymeric material (e.g., polyolefins such as polyethylene orpolypropylene, ethylene vinyl acetate, polyurethanes, polyesters such aspolyethylene terephthalate, and the like). At least some release linersare coated with a layer of a release agent such as a silicone-containingmaterial or a fluorocarbon-containing material. Exemplary release linersinclude, but are not limited to, liners commercially available from CPFilm (Martinsville, Va.) under the trade designation “T-30” and “T-10”that have a silicone release coating on polyethylene terephthalate film.The liner can have a microstructure on its surface that is imparted tothe adhesive to form a microstructure on the surface of the adhesivelayer. The liner can then be removed to expose an adhesive layer havinga microstructured surface.

In many transfer tape embodiments, it is desirable that the transfertape be hand tearable, that is to say that the dispensed adhesive can betorn by hand without the need for cutting of the transfer tape. This isparticularly true when the transfer tape is dispensed from a bladelesshand held dispenser, such as the SCOTCH ATG dispensers commerciallyavailable from 3M Company, St. Paul, Minn. The pressure sensitiveadhesives of the present disclosure not only have the handling strengthrequired of transfer tape, but also are typically hand tearable.

Accordingly, and in another aspect, the present invention relates to theuse of a radiation crosslinkable pressure sensitive adhesive precursoras above described, for the manufacture of an adhesive tape, preferablya double-sided tape.

Item 1 is a radiation crosslinkable pressure sensitive adhesiveprecursor comprising:

-   -   a) an acrylate base polymer;    -   b) a co-polymerized type (II) photocrosslinker, in a amount        greater than 0.05 parts by weight per 100 parts by weight of        acrylate base polymer;    -   c) a co-polymerized hydrogen-donating monomer; and    -   d) a tackifying resin, in an amount greater than 40 parts by        weight per 100 parts by weight of acrylate base polymer.

Item 2 is the precursor of item 1, wherein the amount of co-polymerizedtype (II) photocrosslinker is in amount greater than 0.10 parts, orgreater than 0.15 parts by weight per 100 parts by weight of acrylatebase polymer.

Item 3 is a precursor according to any of item 1 or 2, wherein theamount of acrylate base polymer and co-polymerized hydrogen-donatingmonomer are selected such as to provide the radiation crosslinkedpressure sensitive adhesive obtained by radiation crosslinking,preferably UV radiation crosslinking, the radiation crosslinkablepressure sensitive adhesive precursor, with a static shear at 70° C. ofat least 2000 minutes, preferably at least 4000 minutes, more preferablyat least 6000 minutes, even more preferably at least 8000 minutes, stillmore preferably at least 10000 minutes, when measured according tostatic shear test ASTM D3654.

Item 4 is a precursor according to item 3, wherein the static shear at70° C. is measured on a UV crosslinked pressure sensitive adhesive layercoated on a substrate, wherein the thickness of the pressure sensitiveadhesive layer is typically of about 100 μm, and wherein the UVcrosslinking is typically operated with 250 mJ/cm² UV-B light.

Item 5 is a precursor according to any of the preceding items, whereinthe amount of tackifying resin is greater than 45, preferably greaterthan 50, more preferably greater than 55, even more preferably greaterthan 60 parts by weight per 100 parts by weight of acrylate basepolymer.

Item 6 is a precursor according to any of the preceding items, whichcomprises:

-   -   a) an acrylate base polymer;    -   b) from 0.06 to 1 parts, from 0.11 to 1 parts, from 0.16 to 1        parts, from 0.18 to 0.70 parts, or even from 0.20 to 0.50 parts        by weight per 100 parts by weight of acrylate base polymer, of a        co-polymerized type (II) photocrosslinker;    -   c) from 0.05 to 30 parts, from 0.05 to 10 parts, from 0.05 to 5        parts, from 0.10 to 3 parts, or even from 0.15 to 2 parts by        weight per 100 parts by weight of acrylate base polymer, of a        co-polymerized hydrogen-donating monomer; and    -   d) from 40 to 80 parts, from 40 to 60 parts, or even from 50 to        60 parts by weight per 100 parts by weight of acrylate base        polymer, of a tackifying resin.

Item 7 is a precursor according to any of the preceding items, whichcomprises:

-   -   a) from 45 to 80 wt %, from 45 to 75 wt %, from 50 to 70 wt %,        or even from 60 to 70 wt % by weight of the precursor, of an        acrylate base polymer;    -   b) from 0.01 to 1 wt %, from 0.03 to 1 wt %, from 0.05 to 1 wt        %, from 0.10 to 0.60 wt %, or even from 0.12 to 0.25 wt % by        weight of the precursor, of a co-polymerized type (II)        photocrosslinker;    -   c) from 0.02 to 17 wt %, from 0.02 to 10 wt %, from 0.02 to 4 wt        %, from 0.05 to 2 wt %, or even from 0.08 to 1.5 wt % by weight        of the precursor, of a co-polymerized hydrogen-donating monomer;        and    -   d) from 20 to 50 wt %, from 20 to 45 wt %, or even from 25 to 40        wt % by weight of the precursor, of a tackifying resin.

Item 8 is a precursor according to any of the preceding items, whereinthe acrylate base polymer is obtained from a polymerizable mixturecomprising at least one linear or branched alkyl (meth)acrylate monomer,wherein the linear or branched alkyl group of the alkyl (meth)acrylatemonomer preferably comprises from 1 to 24, more preferably from 4 to 20,even more preferably 6 to 15, still more preferably from 6 to 10 carbonatoms.

Item 9 is a precursor according to item 8, wherein the linear orbranched alkyl (meth)acrylate monomer is selected from the groupconsisting of methyl acrylate, ethyl acrylate, n-propyl acrylate,isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-pentylacrylate, iso-pentyl acrylate, n-hexyl acrylate, iso-hexyl acrylate,cyclohexyl acrylate, octyl acrylate, iso-octyl acrylate, 2-ethylhexylacrylate, decyl acrylate, lauryl acrylate, 2-propylheptyl acrylate,stearyl acrylate, isobornyl acrylate, and any combinations or mixturesthereof.

Item 10 is a precursor according to any of item 8 or 9, wherein thealkyl (meth)acrylate monomer is selected from the group consisting ofiso-octyl acrylate, 2-ethylhexyl acrylate, butyl acrylate, and anycombinations or mixtures thereof.

Item 11 is a precursor according to any of the preceding items, whereinthe acrylate base polymer comprises a co-polymer of at least one alkyl(meth)acrylate monomer which as a homopolymer has a Tg of less than 20°C. and a co-monomer which as a homopolymer has a Tg of greater than 20°C., and wherein the co-monomer which as a homopolymer has a Tg ofgreater than 20° C. is preferably selected to comprise acrylic acid.

Item 12 is a precursor according to any of the preceding items, whereinthe acrylate base polymer comprises a further co-polymerizedphotocrosslinker, preferably a co-polymerized type (II)photocrosslinker.

Item 13 is a precursor according to any of the preceding items, whereinthe acrylate base polymer comprises a co-polymer of iso-octyl acrylate,2-ethylhexyl acrylate, or butyl acrylate and acrylic acid.

Item 14 is a precursor according to any of the preceding items, whereinthe co-polymerized type (II) photocrosslinker is selected from the groupof mono-ethylenically unsaturated aromatic ketones, and is preferablyselected from the group consisting of para-acryloxybenzophenone (ABP),para-acryloxyethoxybenzophenone (AEBP),para-N-(methylacryloxyethyl)-carbamoylethoxybenzophenone,4-acryloyloxydiethoxy-4-chlorobenzophenone, para-acryloxyacetophenone,ortho-acrylamidoacetophenone, acrylated anthraquinones, and anycombinations or mixtures thereof.

Item 15 is a precursor according to any of the preceding items, whereinthe co-polymerized type (II) photocrosslinker is selected from the groupof para-acryloxybenzophenone (ABP), para-acryloxyethoxybenzophenone(AEBP), and any combinations or mixtures thereof.

Item 16 is a precursor according to any of the preceding items, whereinthe co-polymerized hydrogen-donating monomer is selected from the groupof monomers comprising at least one hydrogen atom on a carbon atom in aα-position to a nitrogen, oxygen or sulfur atom. More preferably, theco-polymerized hydrogen-donating monomer is selected from the groupconsisting of (meth)acrylamide, (meth)acrylic, urethane (meth)acrylic,styrenic, and vinylic monomers containing at least one amine functionalgroup, preferably a tertiary amine functional group.

Item 17 is a precursor according to any of the preceding items, whereinthe co-polymerized hydrogen-donating monomer is selected from the groupconsisting of N,N-dimethyl acrylamide; N-vinyl caprolactam; N-Vinylpyrrolidone; N-isopropyl acrylamide; N,N-dimethylaminoethylmethacrylate; 2-[[(Butylamino)carbonyl]oxy]ethyl acrylateN,N-dimethylaminopropyl methacrylamide; N,N-diethylaminopropylmethacrylamide; N,N-dimethylaminoethyl acrylate; N,N-diethylaminoethylacrylate; N,N-dimethylaminopropyl acrylate; N,N-diethylaminopropylacrylate; N,N-di ethylaminoethyl methacrylate; N,N-dimethylaminoethylacrylamide; N,N-dimethylaminoethyl methacrylamide; N,N-diethylaminoethylacrylamide; N,N-diethylaminoethyl methacrylamide; and any combinationsor mixtures thereof.

Item 18 is a precursor according to any of the preceding items, whereinthe co-polymerized hydrogen-donating monomer is selected from the groupconsisting of N,N-dimethyl acrylamide; N-vinyl caprolactam; N-isopropylacrylamide; N,N-dimethylaminoethyl methacrylate;2-[[(Butylamino)carbonyl]oxy]ethyl acrylate and any combinations ormixtures thereof.

Item 19 is a precursor according to any of the preceding items, whereinthe co-polymerized type (II) photocrosslinker is co-polymerized with theacrylate base polymer.

Item 20 is a precursor according to any of the preceding items, whereinthe co-polymerized type (II) photocrosslinker is co-polymerized with acrosslinking polymer distinct from the acrylate base polymer.

Item 21 is a precursor according to any of the preceding items, whereinthe co-polymerized hydrogen-donating monomer is co-polymerized with theacrylate base polymer.

Item 22 is a precursor according to any of the preceding items, whereinthe co-polymerized hydrogen-donating monomer is co-polymerized with acrosslinking polymer distinct from the acrylate base polymer.

Item 23 is a precursor according to any of the preceding items, whereinthe co-polymerized type (II) photocrosslinker and the co-polymerizedhydrogen-donating monomer are co-polymerized with the acrylate basepolymer.

Item 24 is a precursor according to any of the preceding items, whereinthe co-polymerized type (II) photocrosslinker and the co-polymerizedhydrogen-donating monomer are co-polymerized with a crosslinking polymerdistinct from the acrylate base polymer.

Item 25 is a precursor according to any of items 20 to 24, wherein thecrosslinking polymer is an acrylate crosslinking polymer.

Item 25 is a precursor according to any of items 20 to 24, wherein thecrosslinking polymer is obtained from polymerizable mixture comprisingat least one linear or branched alkyl (meth)acrylate monomer, whereinthe linear or branched alkyl group of the alkyl (meth)acrylate monomerpreferably comprises from 1 to 24, more preferably from 4 to 20, evenmore preferably 6 to 15, still more preferably from 6 to 10 carbonatoms.

Item 26 is a precursor according to item 25, wherein the linear orbranched alkyl (meth)acrylate monomer is selected from the groupconsisting of methyl acrylate, ethyl acrylate, n-propyl acrylate,isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-pentylacrylate, iso-pentyl acrylate, n-hexyl acrylate, iso-hexyl acrylate,cyclohexyl acrylate, octyl acrylate, iso-octyl acrylate, 2-ethylhexylacrylate, decyl acrylate, lauryl acrylate, 2-propylheptyl acrylate,stearyl acrylate, isobornyl acrylate, and any combinations or mixturesthereof.

Item 27 is a precursor according to any of items 20 to 26, wherein thecrosslinking polymer is obtained from a polymerizable mixture comprisingmonomers selected from the group consisting of acrylic monomers, vinylester monomers, acryl amide monomers, alkyl (meth)acryl amide monomers,dialkyl acryl amide, styrenic monomers, and any combinations or mixturesthereof.

Item 28 is a precursor according to any of items 20 to 27, wherein thecrosslinking polymer is obtained from a polymerizable mixture which isfree of acidic monomers, preferably free of acrylic acid monomers.

Item 29 is a precursor according to any of the preceding items, whereinthe tackifying resin is selected from the group consisting of terpenephenolics, rosins, rosin esters, esters of hydrogenated rosins,synthetic hydrocarbon resins, and any combinations or mixtures thereof.

Item 30 is a precursor according to any of the preceding items, whereinthe tackifying resin is selected from the group of esters ofhydrogenated rosins.

Item 31 is a precursor according to any of the preceding items, which ishot melt processable.

Item 32 is a radiation crosslinked pressure sensitive adhesiveobtainable by radiation crosslinking the precursor according to any ofitems 1 to 31.

Item 33 is a radiation crosslinked pressure sensitive adhesive accordingto item 32 coated on a substrate thereby forming a crosslinked pressuresensitive adhesive layer, wherein the radiation crosslinked pressuresensitive adhesive layer preferably has a thickness of at least 40 μm,or greater than 50 μm, greater than 70 μm, greater than 80 μm, greaterthan 90 μm, or even greater than 100 μm.

Item 34 is a radiation crosslinked pressure sensitive adhesive accordingto any of item 32 or 33 which is hot melt processable.

Item 35 is a method of preparing a radiation crosslinked pressuresensitive adhesive, comprising the steps of:

-   -   a) providing a radiation crosslinkable pressure sensitive        adhesive precursor according to any of items 1 to 31; and    -   b) radiation crosslinking the radiation crosslinkable pressure        sensitive adhesive precursor.

Item 36 is the method of item 35, comprising the steps of:

-   -   a) providing an acrylate base polymer;    -   b) providing a co-polymerized type (II) photocrosslinker, in a        amount greater than 0.05, typically greater than 0.10, more        typically greater than 0.15 parts by weight per 100 parts by        weight of acrylate base polymer;    -   c) providing a co-polymerized hydrogen-donating monomer;    -   d) providing a tackifying resin, in an amount greater than 40        parts by weight per 100 parts by weight of acrylate base        polymer;    -   e) mixing the acrylate base polymer, the co-polymerized        type (II) photocrosslinker, the co-polymerized hydrogen-donating        monomer and the tackifying resin thereby forming a radiation        crosslinkable pressure sensitive adhesive precursor; and    -   f) radiation crosslinking, preferably UV radiation crosslinking,        the radiation crosslinkable pressure sensitive adhesive        precursor thereby forming a radiation crosslinked pressure        sensitive adhesive.

Item 37 is a method according to any of items 35 or 36, wherein theco-polymerized type (II) photocrosslinker is co-polymerized with theacrylate base polymer.

Item 38 is a method according to any of items 35 to 37, wherein theco-polymerized type (II) photocrosslinker is co-polymerized with acrosslinking polymer distinct from the acrylate base polymer.

Item 39 is a method according to any of items 35 to 38, wherein theco-polymerized hydrogen-donating monomer is co-polymerized with theacrylate base polymer.

Item 40 is a method according to any of items 35 to 39, wherein theco-polymerized hydrogen-donating monomer is co-polymerized with acrosslinking polymer distinct from the acrylate base polymer.

Item 41 is a method according to any of items 35 to 40, wherein theco-polymerized type (II) photocrosslinker and the co-polymerizedhydrogen-donating monomer are co-polymerized with the acrylate basepolymer.

Item 42 is a method according to any of items 35 to 41, wherein theco-polymerized type (II) photocrosslinker and the co-polymerizedhydrogen-donating monomer are co-polymerized with a crosslinking polymerdistinct from the acrylate base polymer.

Item 43 is a method according to any of items 37 to 42, wherein thecrosslinking polymer is an acrylate crosslinking polymer.

Item 44 is a method according to any of items 35 to 43, wherein theamount of acrylate base polymer and co-polymerized hydrogen-donatingmonomer are selected such as to provide the radiation crosslinkedpressure sensitive adhesive obtained by radiation crosslinking,preferably UV radiation crosslinking, the radiation crosslinkablepressure sensitive adhesive precursor, with a static shear at 70° C. ofat least 2000 minutes, preferably at least 4000 minutes, more preferablyat least 6000 minutes, even more preferably at least 8000 minutes, stillmore preferably at least 10000 minutes, when measured according tostatic shear test ASTM D3654.

Item 45 is the method of item 44, wherein the static shear at 70° C. ismeasured on a UV crosslinked pressure sensitive adhesive layer coated ona substrate, wherein the thickness of the pressure sensitive adhesivelayer is of about 100 μm, and wherein the UV crosslinking is typicallyoperated with 250 mJ/cm² UV-B light.

Item 46 is a method according to any of items 35 to 45, comprising thestep of melt mixing the acrylate base polymer, the co-polymerized type(II) photocrosslinker, the co-polymerized hydrogen-donating monomer andthe tackifying resin thereby forming a hot melt processable radiationcrosslinkable pressure sensitive adhesive precursor.

Item 47 is the use of a radiation crosslinkable pressure sensitiveadhesive precursor according to any of items 1 to 31 for the manufactureof an adhesive tape, preferably a double-sided tape.

EXAMPLES

The invention is further illustrated by the following examples. Theseexamples are merely for illustrative purposes only and are not meant tobe limiting on the scope of the appended claims.

Test Methods

1. Static Shear According to ASTM D 3564 (Procedure A)

Shear Strength on Stainless Steel (SS):

This test method determines the ability of pressure-sensitive adhesivetapes to remain adhered under constant load applied parallel to thesurface of the tape and substrate.

Prior to testing all samples are conditioned at ambient conditions (23°C.+/−2° C. and 50%+/−5% relative humidity).

The shear strength is determined following ASTM Designation: D 3654/D3654M-06.

First 1 inch (2.54 cm) wide strip of adhesive is cut from the tape byusing a specimen cutter holding two single-edge razor blades in parallelplanes, the blades spaced 1 inch (2.54 cm) apart. The adhesive strip isthen placed onto a clean, stainless steel panel with bright annealedfinish in accordance with Specification ASTM A666 having a dimension of50 mm by 125 mm (and a minimum thickness of 1.1 mm), covering a 1 inchby 1 inch (2.54 cm×2.54 cm) area of the stainless steel panel. Theadhesive strip is then over-rolled twice in each direction using ahand-held rubber-covered 2 kg hand-roller at a approximate rate of 10mm+/−0.4 mm/s. A 500 gram weight is then used as the static load, andthe test samples are placed on an automated timing apparatus in a aircirculated oven at 70° C. The mode of failure for all samples iscohesive failure. The data is reported as an average of threemeasurements.

2. 180° Peel Adhesion According to ASTM Designation: D3330/D330M-04

This test method covers the measurement of the peel adhesion ofpressure-sensitive adhesive tapes. Peel adhesion is measured againststainless steel and PE covered aluminum panels as test substrates. Thestainless steel panels have the following dimensions: 50 mm by 125 mmand a minimum thickness of 1.1 mm. The PE covered aluminum panels have adimension of 50 mm by 150 mm and a thickness of 2 mm.

In case of the stainless steel panels, the panels need to conform toType 302 or 304 of Specification ASTM A 666, having a bright annealedfinish. The PE covered aluminum panels are made by selecting a 13 milsthick (330 micrometers) polyethylene film with similar dimensions as thealuminum panel, the PE film made from polyethylene (PE) pellets beingavailable under trade designation “VORIDIAN POLYETHYLENE 1550P” fromEastman Chemical Co. (Kingsport, Tenn., USA), and fixing the PE filmwith an adhesive film to the aluminum plate. Test surface of the PEselected is the rougher surface side.

Prior to testing all samples are conditioned at ambient conditions (23°C.+/−2° C. and 50%+/−5% relative humidity).

In a climate room set at ambient conditions (23° C.+/−2° C. and 50%+/−5%relative humidity), 1 by 1 inch (2.54 cm) wide adhesive strips having alength of approximately 300 mm are cut from the conditioned samplesusing a specimen cutter holding two single-edged razor blades inparallel planes of the adhesive. The strip is then placed withoutpressure onto either a cleaned, stainless steel panel or a PE panel.Cleaning of the stainless steel panels is done by wiping the panels witha lint free tissue first with a pass of methyl ethyl ketone (MEK),followed by a wipe with n-heptane and finally another pass with methylethyl ketone (MEK). Wiping of the panels per pass of solvent is alwaysdone until dryness.

The adhesive strips are then laminated onto the substrate using a 2 kghand-held rubber-covered roller at a rate of 10+/−0.5 mm/s with 2 passesin each direction. After a dwell time of 20 minutes in the climate room,a 180° peel test is performed using a FP-2255 Peel Tester (manufacturedby Thwing-Albert Instrument Company), with data collected and averagedover 10 seconds. Three measurements are made per example and the averagerecorded in N/inch.

List of materials Name UIPAC Name Supplier CAS nr NNDMAN,N-Dimethylacrylamide TCI 2680-03-7 NVC Vinylcaprolactam BASF 2235-00-9DMAEMA N,N-Dimethylaminoethyl Evonik 220-688-8 methacrylate EOEA2-Ethoxyethyl acrylate Aldrich 106-74-1 NiPAm N-IsopropylacrylamideAldrich 2210-25-5 Ebecryl CL 2-[[(Butylamino)carbon- Cytec 63225-53-61039 yl]oxy]ethyl acrylate PPG- Poly(ethylene glycol) acrylate Aldrich9051-31-4 Acrylate IOA Isooctyl acrylate 3M 29590-42-9 AA Acrylic acidBASF 79-10-7 HEA Hydroxy ethyl acrylate DOW 818-61-1 ABP4-Acryloxybenzophenone 3M 22535-49-5 AEBP para-acryloxyethoxybenzo- 3M —phenone V 601 Dimethyl-2,2′- Wako 2589-57-3 azobisisobutyrate ChemicalsGmbH Ethyl Ethyl Acetate DOW 141-78-6 Acetate Heptane Heptane Exxon142-82-5 PETMP Pentaerythritol tetrakis(3- Aldrich 7575-23-7mercaptopropionate) IOTG Isooctylthioglycolate Arkema 25103-09-7Irgacure 651 2,2-Dimethoxy-1,2- Ciba 24650-42-8 diphenylethan-1-oneVeova 10 Vinyl ester of neodecanoic Momentive 51000-52-3 acid Foral 85Ea glycerol ester of highly Eastman hydrogenated refined gum rosinMaking of Base Polymers:Preparation of Base Polymer B0 (Hot Melt Pressure Sensitive Adhesive):

Base polymer B0 has a relatively low molecular weight and its ABPcontent is chosen in such a way to avoid gelation in the pouches.

A copolymer of IOA and AA is bulk polymerized under UV light sealed inethylene vinyl acetate film pouches as described in U.S. Pat. No.6,294,249 (Hamer et al.). Two sheets of 2.5 mil (51 micrometer) thickethylene vinyl acetate, commercially available as VA-24 from PliantCorp. of Evansville, Ind., are heat sealed on the lateral edges and thebottom to form a rectangular pouch on a liquid form, fill, and sealmachine. The pouch is filled with a pre-adhesive composition having 96parts IOA, 4 parts AA, 0.20 phr of Irgacure 651, 0.10 phr ABP and 0.04IOTG. The filled package is then heat sealed at the top in the crossdirection through the monomer to form individual pouches measuring 13.4cm by 4.3 cm by about 0.4 cm thick containing 25 grams of thepre-adhesive composition. The pouches are placed in a water bath that ismaintained between about 16° C. and 32° C. and exposed to ultravioletradiation (supplied by lamps having about 90 percent of the emissionsbetween 300 and 400 nanometers (nm), and a peak emission at 351 nm) atan intensity of 4.55 mW/cm² for 21 minutes.

Making of Base Polymers B1-B7:

Further base polymers are produced via solution polymerization.Typically a 45 wt % solution of an acrylate mixture (mostly IOA/AA) in a85:15 parts solvent mixture of ethyl acetate and heptane is prepared.The polymerization is started by an azo initiator (V 601) and themixture polymerized under constant stirring for 20 hours at 60° C.Formulations of all used base polymers are provided in Table 1:

TABLE 1 Base polymer IOA AA NNDMA HEA ABP B0 96 4 0.1 B1 96 4 0.2 B295.8 4 0.2 B3 94 4 2 0.25 B4 95.85 4 0.15 0.25 B5 94 4 2 0 B6 96 4 0.1B7 96 4 0.05Making of Polymeric Crosslinkers XL0-XL 14

The crosslinking polymers used for the examples are produced in solutionand for the later described hot melt experiments a so-called polymelt iscreated by stripping the solvent. Typically a 50 wt % solution of anacrylate mixture in ethyl acetateis prepared. The molecular weight iscontrolled by adding as chain transfer agent pentaerythritoltetrakis(3-mercaptopropionate) (PETMP). The polymerization is started byan azo initiator (Vazo 601) and polymerization takes place underconstant stirring for 20 hours at 65° C. Table 2 shows an overview ofall crosslinking polymers used for the examples.

TABLE 2 Crosslinking polymer IOA Veova HDM ⁽*⁾ Amount AEBP XL0 72.5 17.5NNDMA 5 5 XL1 95 5 XL2 60 NNDMA 40 XL3 90 AA 5 5 XL4 72.5 17.5 NNDMA 5 5XL5 77.5 17.5 5 XL6 71.8 17.5 NiPAm 5.7 5 XL7 68 20 NVC 7 5 XL8 70.317.5 EOEA 7.2 5 XL9 72.5 17.5 Ebecryl CL 1039 5 5 XL10 72.5 17.5 PPGAcrylate 5 5 XL11 70 15 DMAEMA 10 5 XL12 70 15 DMAEMA 10 5 XL13 75 17.5NNDMA 5 2.5 XL14 0 0 NNDMA 97.5 2.5 ⁽*⁾ Hydrogen-donating monomer

For all examples made from the base polymers and the crosslinkingpolymers, a hydrogenated rosin ester known as Foral 85E (commerciallyavailable from Eastman Chemical Company) is used as tackifier.

Making of Examples Ex 1-Ex 22 (Solution) and HM 1-HM 3 (Hot Melt):

All examples, made from solution or via hot melt processing, are coatedto a thickness of 100 μm. For the hot melt examples the base polymer B0is used and crosslinking polymers XL0 or XL4 in combination with thetackifying resin Foral 85E. Hotmelt compounding is run in a twin screwextruder from Werner & Pfleiderer (Stuttgart, Germany) at 160° C. and ascrew speed of 320 rpm. The hot melt coating itself is done using arotary rod die and the samples are cured offline with 250 mJ/cm² UV-Blight under a medium pressure mercury lamp (available from TCSTechnologies). The liner used is a paper liner, siliconized on bothsides (available from Mondi Akrosil, Wisconsin, USA), and having athickness of 75 μm.

Table 3 shows the hot melt processed examples HM 1 through HM 3.

TABLE 3 HM formulation Base polymer Parts Crosslinker Parts Foral E HM 1B0 100 XL0 3.0 50 HM 2 B0 100 XL0 6.0 50 HM 3 B0 100 XL4 3.0 50

Examples 1-24 are prepared by blending the components in solution underconstant mixing at room temperature (23° C.+/−2° C.) for 60 minutes at10 rpm/min. The mixtures are then knife coated on a white, double-sidedsiliconized paper liner (available from Mondi akrosil, USA) having athickness of 75 μm. Coatings 1-23 have a thickness of 100 μm+/−5 μm andcoating 24 has a thickness of 50 μm+/−2 μm. The coatings are irradiatedwith 250 mJ/cm² UV-B light under medium pressure mercury lamp (availablefrom TCS Technologies). For testing, the coatings are laminated on 50 μmaluminized PET liner (commercially available as metalized Hostaphan RNfrom Mitsubishi Polyester Film GmbH, Wiesbaden, Germany) and testspecimen are prepared for the static shear and 180° peel measurements aspreviously described.

Table 4 provides an overview of examples Ex1-Ex 24:

Example Base Crosslinking Parts of tackifying No. polymer Parts polymerParts resin (Foral 85E) Ex 1 B1 100 — — 50 Ex 2 B0 100 XL 1 3.0 50 Ex 3B0 100 XL 4 3.0 50 Ex 4 B2 100 XL 2 3.0 50 Ex 5 B5 100 XL 3 7.5 50 Ex 6B3 100 — — 50 Ex 7 B4 100 — — 50 Ex 8 B0 100 XL 5 3.0 50 Ex 9 B0 100 XL6 3.0 50 Ex 10 B0 100 XL 7 3.0 50 Ex 11 B0 100 XL 8 7.5 50 Ex 12 B0 100XL 9 7.5 50 Ex 13 B0 100 XL 10 7.5 50 Ex 14 B0 100 XL 11 3.0 50 Ex 15 B6100 XL 5 3.0 50 Ex 16 B6 100 XL 4 3.0 50 Ex 17 B6 100 XL 6 3.0 50 Ex 18B6 100 XL 12 3.0 50 Ex 19 B0 100 XL 4 3.2 60 Ex 20 B0 100 XL 4 4.8 60 Ex21 B0 100 XL 4 6.4 60 Ex 22 B0 100 XL 4 16 60 Ex 23 B0 100 XL 14 3.0 50Ex 24 B7 100 XL 13 2.8 40Test Results

As previously disclosed, the hydrogen donating monomer can be eitherincorporated into the base polymer or the crosslinking polymer, or intoboth. The same statement applies for the co-polymerized type (II)photocrosslinkers, which can be co-polymerized into the acrylate basepolymer or co-polymerized into a crosslinking polymer, or into both.Table 5 shows the test results of 180° peel adhesion to stainless steeland PE panels as also the static shear values at 70° C. on stainlesssteel of examples Ex 1-Ex 7. These examples are set-up for showing theeffects of the hydrogen donor and the photocrosslinkers depending on ifthey are part of the base polymer or the crosslinking polymer.

TABLE 5 180° peel to steel 180° peel to PE Static shear at 70° C.Example No (N/inch) (N/inch) (min) Ex 1 — — 247 Ex 2 — — 17 Ex 3 23.014.4 +10000 Ex 4 18.0 13.8 +10000 Ex 5 20.6 12.9 +10000 Ex 6 23.3 15.4+10000 Ex 7 24.1 14.8 +10000 Ex 23 17.2 16.8 +10000 Ex 24 16.1 17.5+10000

The results from Table 5 show that formulations Ex 1 and Ex 2, which donot contain any hydrogen donating monomer in the formulation, have apoor shear performance.

Ex 2 and Ex 3 have the same amount of benzophenone in the formulationbut in Ex 3, N,N dimethyl acrylamide (NNDMA) is incorporated as ahydrogen donating monomer (HDM) in the crosslinking polymer. A bigincrease in shear performance can be observed for Ex 3.

Further, Ex 1 and Ex 4 have an equal concentration of benzophenone. InEx 4, the crosslinking polymer is a IOA/NNDMA copolymer. Adding thishigh amount of HDM to the mixture generates the same increase in shearperformance. Ex 5 shows that also the opposite is working.

The hydrogen donating monomer is now incorporated in the base polymerand all the benzophenone is located in the crosslinker. Ex 6 and Ex 7show that the system also works when ABP and the HDM are located in thesame base polymer. Ex 23 uses a crosslinker which is poly(N,NDimethylacrylamide) based. Also, when the total amount ofphotocrosslinker is reduced as shown in Ex 23 and Ex 24, good shearperformance is still obtained.

Upper examples (Ex 3 to Ex 7, as well as Ex 23 and Ex 24) have in commonthat they use as hydrogen donor NNDMA. Following test results, asprovided in Table 6, show the influence of different kinds of hydrogendonors on the shear and 180° peel values.

TABLE 6 180° peel to steel 180° peel to PE Static Shear at 70° ExampleNo (N/inch) (N/inch) (min) Ex 8 — — 3 Ex 9 20.9 13.7 +10000 Ex 10 21.215.7 +10000 Ex 12 21.0 11.2 +10000 Ex 14 19.6 12.9 +10000

In following Table 7, highly tackified PSA systems containing 60 partsForal 85E are evaluated.

TABLE 7 180° peel to steel 180° peel to PE Static Shear at 70° C.Example no (N/inch) (N/inch) (min) Ex 19 25.8 13.9 9688 Ex 20 24.3 14.5+10000 Ex 21 22.7 12.3 +10000 Ex 22 20 11.7 +10000

In Table 8, the results of the hot melt examples are shown.

TABLE 8 180° peel to 180° peel Static Shear HM stainless steel to PE at70° C. formulation (N/inch) (N/inch) (min) HM 1 26.0 13.8 4343 HM 2 25.212.7 +10000 HM 3 26.1 14.0 +10000

We claim:
 1. A radiation crosslinkable pressure sensitive adhesiveprecursor comprising: I. an acrylate base polymer comprising: a) aco-polymerized type (II) photocrosslinker, in an amount greater than0.05 parts by weight per 100 parts by weight of acrylate base polymer;b) a co-polymerized hydrogen-donating monomer selected from the groupconsisting of (meth)acrylamide, (meth)acrylic, urethane (meth)acrylic,styrenic, and vinylic monomers containing at least one amine functionalgroup and any combinations or mixtures thereof; and c) co-polymerizedacrylic acid in an amount of 0.5 to 25 parts by weight per 100 parts byweight of acrylate base polymer; and II. a tackifying resin, in anamount greater than 40 parts by weight per 100 parts by weight ofacrylate base polymer.
 2. The precursor according to claim 1, whereinthe amount of co-polymerized type (II) photocrosslinker is in an amountgreater than 0.10 parts by weight per 100 parts by weight of acrylatebase polymer.
 3. The precursor according to claim 1, wherein the amountof acrylate base polymer and co-polymerized hydrogen-donating monomerare selected such as to provide the radiation crosslinked pressuresensitive adhesive obtained by radiation crosslinking the radiationcrosslinkable pressure sensitive adhesive precursor, with a static shearat 70° C. of at least 2000 minutes when measured according to staticshear test ASTM D3654.
 4. The precursor according to claim 3, whereinthe static shear at 70° C. is measured on a UV crosslinked pressuresensitive adhesive layer coated on a substrate, wherein the thickness ofthe pressure sensitive adhesive layer is about 100 μm, and wherein theUV crosslinking is operated with 250 mJ/cm² UV-B light.
 5. The precursoraccording to claim 1, wherein the amount of tackifying resin is greaterthan 45 parts by weight per 100 parts by weight of acrylate basepolymer.
 6. The precursor according to claim 1, wherein the amount oftackifying resin is greater than 55 parts by weight per 100 parts byweight of acrylate base polymer.
 7. The precursor according to claim 1,wherein the amount of tackifying resin is greater than 60 parts byweight per 100 parts by weight of acrylate base polymer.
 8. Theprecursor according to claim 1, wherein the co-polymerized type (II)photocrosslinker is selected from the group consisting ofmono-ethylenically unsaturated aromatic ketones and any combinations ormixtures thereof.
 9. The precursor according to claim 1, wherein theco-polymerized type (II) photocrosslinker is selected from the groupconsisting of para-acryloxybenzophenone (ABP),para-acryloxyethoxybenzophenone (AEBP),para-N-(methylacryloxyethyl)-carbamoylethoxybenzophenone,4-acryloyloxydiethoxy-4-chlorobenzophenone, para-acryloxyacetophenone,ortho-acrylamidoacetophenone, acrylated anthraquinones, and anycombinations or mixtures thereof.
 10. The precursor according to claim1, wherein the co-polymerized hydrogen-donating monomer is selected fromthe group consisting of N,N-dimethyl acrylamide; N-vinyl caprolactam;N-Vinyl pyrrolidone; N-isopropyl acrylamide; N,N-dimethylaminoethylmethacrylate; 2-[[(Butylamino)carbonyl]oxy]ethyl acrylateN,N-dimethylaminopropyl methacrylamide; N,N-diethylaminopropylmethacrylamide; N,N-dimethylaminoethyl acrylate; N,N-diethylaminoethylacrylate; N,N-dimethylaminopropyl acrylate; N,N-diethylaminopropylacrylate; N,N-diethylaminoethyl methacrylate; N,N-dimethylaminoethylacrylamide; N,N-dimethylaminoethyl methacrylamide; N,N-diethylaminoethylacrylamide; N,N-diethylaminoethyl methacrylamide; and any combinationsor mixtures thereof.
 11. A method of preparing a radiation crosslinkedpressure sensitive adhesive, comprising the steps of: providing anacrylate base polymer comprising: a) a co-polymerized type (II)photocrosslinker, in an amount greater than 0.05 parts by weight per 100parts by weight of acrylate base polymer; and b) a co-polymerizedhydrogen-donating monomer selected from the group consisting of(meth)acrylamide, (meth)acrylic, urethane (meth)acrylic, styrenic, andvinylic monomers containing at least one amine functional group and anycombinations or mixtures thereof; and c) co-polymerized acrylic acid inan amount of 0.5 to 25 parts by weight per 100 parts by weight ofacrylate base polymer; and providing a tackifying resin, in an amountgreater than 40 parts by weight per 100 parts by weight of acrylate basepolymer; mixing the acrylate base polymer and the tackifying resinthereby forming a radiation crosslinkable pressure sensitive adhesiveprecursor; and radiation crosslinking the radiation crosslinkablepressure sensitive adhesive precursor thereby forming a radiationcrosslinked pressure sensitive adhesive.
 12. The method according toclaim 11, wherein the step of radiation crosslinking is UV crosslinking.13. The method according to claim 11, wherein the amount of acrylatebase polymer and co-polymerized hydrogen-donating monomer are selectedsuch as to provide the radiation crosslinked pressure sensitive adhesiveobtained by radiation crosslinking the radiation crosslinkable pressuresensitive adhesive precursor, with a static shear at 70° C. of at least2000 minutes when measured according to static shear test ASTM D3654.14. The method according to claim 13, wherein the static shear at 70° C.is measured on a UV crosslinked pressure sensitive adhesive layer coatedon a substrate, wherein the thickness of the pressure sensitive adhesivelayer is about 100 μm, and wherein the UV crosslinking is operated with250 mJ/cm² UV-B light.
 15. The method according to claim 11, wherein theamount of tackifying resin is greater than 45 parts by weight per 100parts by weight of acrylate base polymer.
 16. The method according toclaim 11, wherein the amount of tackifying resin is greater than 55parts by weight per 100 parts by weight of acrylate base polymer. 17.The method according to claim 11, wherein the amount of tackifying resinis greater than 60 parts by weight per 100 parts by weight of acrylatebase polymer.
 18. A crosslinked pressure sensitive adhesive obtained byradiation crosslinking the radiation crosslinkable pressure sensitiveadhesive precursor according to claim 1.